A mobile communication system is designed to achieve high communication efficiency with limited resources (e.g., frequency, time) via a multiple access technique. Currently, multiple access technologies are used during implementation of communication systems. Some multiple access technologies include: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiple Access (OFDMA).
Specifically, FDMA provides multiple access to user terminals by splitting frequency resources and allocating them to the user terminals. TDMA provides multiple access to use terminals by splitting time resources and allocating the generated time slots to the user terminals. CDMA provides user terminals with multiple access by allocating each user terminal an orthogonal code to eliminate mutual interference among user terminals. OFDMA provides multiple access to user terminals by splitting and allocating an orthogonal frequency resource. OFDM is a multi-carrier modulation technology suited for high data rate wideband wireless transmission.
Massive multiple-input multiple-output (MIMO) transmission employs a large number of antennas at the base stations to serve several user terminals simultaneously. With the potential large gains in spectral efficiency and energy efficiency, massive MIMO is a promising technology that future wireless systems may incorporate. Due to OFDM's robustness to channel frequency selectivity and relatively efficient implementation, OFDM combined with massive MIMO may be implemented in wideband massive MIMO transmission.
With severe spectrum shortage in the currently deployed cellular bands (sub-6 GHz) and the explosive wireless traffic demand, there is a growing consensus on utilizing higher frequency bands, for example, the millimeter-wave (mmW) band and the Terahertz (THz) band for future wireless communication systems. Combination of massive MIMO with mmW/THz technologies is appealing from a practical point of view. Orders-of-magnitude smaller wavelength in mmW/THz bands enables a larger number of antennas to be deployed at both the user terminals and base stations. Even for a high propagation path loss at mmW/THz channels, the achievable high beamforming gains with massive MIMO may help to compensate for it. Therefore, massive MIMO transmission over mmW/THz bands, which will be referred to as mmW/THz massive MIMO, is envisioned as a solution for future wireless communication systems.
Time and frequency synchronization of the transmission signals may be utilized to stabilize wireless transmission. One synchronization approach for MIMO-OFDM systems may be to compensate for the time/frequency offsets of the received signals in the antenna domain using the time/frequency adjustment parameters. In the scenarios such as high mobility and/or high carrier frequency (e.g., mmW/THz bands), the Doppler spreads of the wireless channels may be increased; meanwhile the channel delay spreads may not vary significantly. In OFDM systems, the cyclic prefix (CP) length may be set to be slightly larger than the delay spread to mitigate channel dispersion in time while the OFDM symbol length may be set to be inversely proportional to the Doppler spread to mitigate channel dispersion in frequency, which may lead to the wireless transmission system bottleneck.
Beamforming performed at the base stations and user terminals may divide the wireless channels in the space, and mitigate the fluctuation of envelopes of the beam domain channel elements. With this property, a per-beam time/frequency synchronization method for wireless transmission is proposed in the present disclosure.